Enzymes that contain manganese are found in organisms ranging from fungi, to plants to humans. Often, manganese remains in the divalent oxidation state functioning to assist hydrolysis reactions. However, there are several cases where manganese, acting in a redox capacity, carries out chemical transformations that either have important human health consequences (e.g., the Mn superoxide dismutase, SOD) or are carrying out unique reactions with global consequences (i.e., the oxygen evolving complex, OEC). This proposal focuses on understanding the structure, physical properties and reactivity of small molecule model compounds for these manganese enzymes that contain 1 to 4 metals with oxidation levels ranging from Mn II to Mn v. Specifically, we propose the following studies: Preparation of compounds with increased nitrogen and carboxylate ligation to mimic the active sites of MnSOD, catalases and arginases, enzymes which have all been structurally characterized. Kinetic analysis of the most active Mn catalase reactivity mimics yet found. Preparation of Mn complexes containing bridging imidazolates to evaluate the weak magnetic exchange limit for tri- and tetranuclear Mn clusters. These compounds may be useful in evaluating spin topologies in the triangle plus one model of manganese organization in PSII. Preparation and characterization of new complexes designed to simultaneously incorporate Mn II and Mn TM. These compounds should model the So state of the OEC. Preparation of dimeric complexes containing Mn(V)=O moieity to model the high oxidation states of the OEC and reactivity studies to understand the factors that control one versus two electron oxidations carried out by these systems. Upon completion of these studies, we will have a better understanding of the active site structure and mechanism of catalysis of a variety of manganese enzymes. These studies will not only answer important questions for manganese biochemistry, but should also provide information which will aid our understanding of health related, non-heme iron enzymes.